On Thursday, March 30, 2017 at 4:14:05 PM UTC-4, Ed M wrote:
replying to someone, Ed M wrote:
How about if you first scribe a line where you want to make your cut, nail a
temporary straight edge to the tread as a circular saw guide. Slowly lower
your circular saw onto the cutting line and cut as far as you can toward the
wall. Finish the cut with an oscillating tool.


13 years later, we have an answer. I'm sure the OP will be very happy to
finally get the job completed.